When using digital copyrighted works such as music, video, video games, etc. (hereinafter, content) on a terminal apparatus, a server which holds the content judges whether the terminal apparatus has usage rights for the content. If judged to have usage rights, the server registers the terminal apparatus and provides the content to the registered terminal apparatus. A technique for judging the status of a content usage right using a credit card number is disclosed in patent document 1, while a technique for judging is performed using a mobile phone in patent document 2.
As one of the above judging techniques, the server transmits a measuring packet to the terminal apparatus, receives a response packet from the terminal apparatus, measures a packet communication time from the transmission of the measuring packet until the reception of the response packet, and judges that the terminal apparatus has usage rights if the measured time is less than or equal to a reference value. This technique is based on the premise that the terminal apparatus has usage rights if a communication time with the server is short, such as a case where the server and the terminal apparatus are in the same house.    Patent Document 1: Japanese Patent Application Publication No. 2002-169726    Patent Document 2: Japanese Patent Application Publication No. 2002-305514